use accelerator for setCheckerboard in RHMC

[edbennett]

This goes some way towards #378. One-flavour RHMC is 10–20% faster with this, and gives identical results (i.e. generated configurations, excluding the header, are bitwise identical).

Things that could potentially be improved:

• I don't understand what the optional checker_dim_half parameter to acceleratorSetCheckerboard does, so I have left it at the default value.
• In principle I imagine that all calls to setCheckerboard could behave the same way; adding a dispatcher function in Lattice_transfer.h that implements the code in this PR in a single place, and then replacing all calls to setCheckerboard with calls to this new function would achieve this. However, I'm not set up to verify that the other calls to setCheckerboard still give correct results after this change, so I took the more conservative approach of just changing what I can confidently claim works.